The following is a rough guide of choosing the right camera to your specific needs!!

-Action:
----------The camera must have a high shutter speed (to capture sharp unblurred images)
----------Ability to take continous shots
----------Large frame rate (eg, 6 upwards is quick)
----------Large autofocus points (the more the better if you wanna autofocus a subject)

-Low light/night time shots
----------Low noise at high ISO levels (once again ISO, noise etc is explained in the sticky thread)

Motorsport/ Action means fast movement (unless said motorsport involves parked cars)
Fast movement requires fast lens if light conditions are not ideal (i.e. cloudy, early morning, late afternoon).
Invest in good lenses too if possible, it'll go a hell lot of a longer way than a good camera body.
Something with f/2.8 as a minimum would be a good start (can be expensive, but third party manufacturers like Sigma, Tamron, Tokina make excellent lenses that aren't pricey)
Big aperture is also sometimes desirable for portrait shots (when you want that creamy blurry background, isolating the subject)

Here's everything you need to know to make your purchase with confidence. ... Psst! This guide is chock full of general information about cameras, lenses, and .... Though action cams have come a long way since they were first introduced